When the light level is low, the pixels of a matrix image sensor collect few electrons. It is necessary to greatly increase the integration time in order to obtain an image, but this is to the detriment of the signal-to-noise ratio.
In CCD (charge-coupled device) technology, it has already been proposed to incorporate, in a sensor, electron multiplication systems that create additional electrons from the electrons naturally generated by the light. The electrical signal which is then received is therefore multiplied by a factor. The noise also increases, but in a lower ratio than the signal.
These electron multiplication principles in CCD technology consist in increasing the potential differences present between the charge transfer gates, thereby accelerating the electrons during transfer. The energy imparted thereto is sufficient for the collisions with the atoms of the semiconductor material to make electrons in these atoms pass from the valence band to the conduction band. These electrons create electron-hole pairs, which charge carriers are themselves accelerated and may give rise to further collisions. The result is an electron multiplication phenomenon.
This phenomenon can occur in CCD sensors since the electrons are transferred from gate to gate and it is the increase in voltage on certain gates that enables the electrons to be greatly accelerated in order to cause this multiplication.
However, in active-pixel sensors, which comprise within each pixel a charge-voltage conversion circuit (a few transistors), multiplication is not possible since the electron packets are voltage-converted immediately after each integration period. The electrons are not transferred from gate to gate. However, active-pixel sensors have already been proposed that use a multi-gate multiplication stage between a photodiode and a charge storage node, within the pixel. However, this stage generates losses due notably to the poor quality of charge transfer in the multiplication gates, except if these gates are mutually overlapping, but then this requires the use of a more costly technology having at least two levels of gates.